1. Field of the Invention
The present invention relates to a toner used for developing electrostatic latent images in electrophotography, electrostatic printing, etc.
2. Description of the Related Art
Recently, in response to demands from offices and common users, downsizing, high speeds, low prices and low energy consumption have been aimed in apparatuses such as copying machines and printers. In particular, while high-quality images have been achieved in ink-jet printers, there are increasing demands for high-quality images also in electrophotographic processes.
With respect to developers used for developing electrostatic latent images in systems, such as electrophotography systems and electrostatic printing systems, some of them are conventionally produced by a kneading-pulverization method and a wet method that is typically exemplified by a suspension polymerization method. Moreover, in order to improve characteristics of particles obtained by these methods, after the particles (developer particles) have been prepared, they are subjected to surface-modifying treatments by various methods (mechanical impact force, heat, etc.).
Taking the surface-improvements into consideration, the inventors, etc. have attempted to improve the toner-quality and performance by controlling the toner shape, and reached the following findings:
For example, by making the toner shape as spherical as possible, the following effects may be achieved:
Since the aggregation between toner particles becomes smaller, good quality free from image losses is obtained;
Good transferring efficiency is obtained because of a high moving properties;
Since external stress is applied uniformly, it is possible to avoid local deformation and degradation (uniformity in quality) in toner;
It is possible to reduce undesired phenomenon, such as aggregation, excessive charging and selective developing (a phenomenon in which toner having a specific particle size or a quantity of charge is first consumed selectively), which are inherent disadvantages of a small-particle toner ingredient;
Since the surface shape becomes more uniform as compared with irregular shaped toner, the distribution of charge density on the toner surface becomes more uniform, thereby providing a sharp distribution in the quantity,of charge;
However, although the above-mentioned effects may be obtained, small aggregating properties and high moving properties (in addition to high fluidity) causes scattering upon transferring process and degradation in the cleaning properties.
Scattering is a serious problem which causes degradation in image quality not only in monochrome images but also in color images, and in particular, gives adverse effects on full color processes in which colors are superimposed. With respect to the cleaning properties, defective cleaning-properties such as passing-through phenomenon of toner particles and unswept toner particles may easily occur, in particular when a cleaning blade is used. When these phenomena occur, it becomes difficult to apply an appropriate potential to the photosensitive member, resulting in a drastic change in the developing characteristics. Serious defects in images, such as white spots, fogs, unevenness and memory (a phenomenon in which the previous image pattern appears in the same period) cause conspicuous degradation in the image quality. These disadvantages appear not only in commonly-used photosensitive members, but also in belt-type photosensitive members and intermediate transferring members.
Moreover, the spherical toner, when used together with even a small amount of a commonly-used fluidizing agent, exhibits a very high fluidizing properties, and tends to cause a problem about packing and sealing (toner leakage) inside the machine.
Together with securing downsizing, high speeds and low-temperature fixing performance of the machine, it becomes necessary to ensure the maintenance for heat resistance and the thermal stability (amount of heat resulting from frictional heat from the regulating section and the machine temperature (temperature rise) due to high ambient temperature and heat generated from the fixing device) inside the machine except for the time of fixing, while maintaining a sufficient fixing properties (quality) by low thermal energy. For this purpose, the effects of uniformity and spherization of the toner shape may be utilized; however, this is not sufficient.
In order to solve the above-mentioned problems, the objective of the present invention is to provide a toner which is suitable for practical use without defects such as insufficient cleaning and scattering, and can achieve a stable image-forming performance for a long time, in addition to the advantage of the toner that is formed into a uniform and spherical shape or a shape close to sphericity.
The present invention relates to a toner, comprising;
toner particles containing at least a binder resin and a colorant, and having an average degree of roundness from 0.960 to 1.0 and a standard deviation of degree of roundness of not more than 0.040; and
silica particles having an average primary particle size of 16 to 28 nm, in which the number of particles (A) that are less than 15 nm in particle size, the number of particles (B) that are between 15 and 30 nm in particle size and the number of particles (C) that are larger than 30 nm is in relation of B/A greater than 4 and B/C greater than 4.